Simultaneous bipotentiostatic electro-deposition of gold onto two closely spaced platinum disc electrodes produces paired gold electrode junctions. The potential applied during gold deposition onto the two electrodes is chosen to be slightly offset (by 0.5 mV) to allow for an automatic cut-off mechanism to control the resulting gap size for the paired diffusion junction. The key parameter controlling the gap size is identified as the deposition potential (in relation to the reversible potential for gold deposition) and paired gold electrode junctions are prepared reproducibly from ca. 5 mu m estimated gap size (-250 mV overpotential during deposition) down to approximately 300 nm estimated gap size (-20 mV overpotential during deposition). The resulting paired junction electrodes are characterized for the reduction of alizarin red S and it is shown that for junctions with ca. 300 nm gap size: (i) the low concentration sensitivity is improved to allow sub-micromolar concentration measurements and (ii) due to faster mass transport within the smaller gap a transition from reversible to irreversible electron transfer can be observed. (C) 2009 Elsevier B.V. All rights reserved.